static bool IsPossibleLockLocation(TileIndex tile)
{
	DiagDirection dir = GetInclinedSlopeDirection(GetTileSlope(tile));
	if (!IsValidDiagDirection(dir)) return false;

	TileIndexDiff delta_mid = TileOffsByDiagDir(dir);
	if (!IsTileFlat(tile + delta_mid)) return false;
	if (!IsTileFlat(tile - delta_mid)) return false;

	return true;
}

/**
 * Tests whether a tile is suitable for a lock for the provided diagonal direction (axis).
 * @param tile The middle tile of a lock
 * @param dir Any diagonal direction belonging to the same axis
 * @param check_tile_flat If set to true, check whether the tile is flat
 * @return true if a lock can be placed in the given direction
 */
bool IsPossibleLockLocationOnDiagDir(TileIndex tile, DiagDirection dir, bool check_tile_flat) {
	if (check_tile_flat && IsTileFlat(tile)) return false;
	DiagDirection tdir = GetInclinedSlopeDirection(GetTileSlope(tile));
	return (tdir == dir || tdir == ReverseDiagDir(dir)) && IsPossibleLockLocation(tile);
}

/**
 * Determine if a player could possibly build a ship lock that would cover this tile.
 *
 * @param bridge_tile The tile to possibly outrule a bridge position
 * @param bridge_dir The bridge direction
 * @return true if a player could possibly build a ship lock on this tile.
 * @see GrowTownWithBridge
 */
static bool IsPossibleLockLocationOnTownBridgeTile(const TileIndex bridge_tile, const DiagDirection bridge_dir)
{
	const DiagDirection dir_side = ChangeDiagDir(bridge_dir, DIAGDIRDIFF_90LEFT);
	const TileIndexDiff delta_side = TileOffsByDiagDir(dir_side);

	if (IsValidTile(bridge_tile) && IsPossibleLockLocationOnDiagDir(bridge_tile, bridge_dir, true) && IsWaterTile(bridge_tile) ||
			IsValidTile(bridge_tile + delta_side) && IsPossibleLockLocationOnDiagDir(bridge_tile + delta_side, dir_side, true) && IsWaterTile(bridge_tile + delta_side) ||
			IsValidTile(bridge_tile - delta_side) && IsPossibleLockLocationOnDiagDir(bridge_tile - delta_side, ReverseDiagDir(dir_side), true) && IsWaterTile(bridge_tile - delta_side)) {
		return true;
	}

	return false;
}